In depth: the technology of time-keeping

How do you set an atomic clock?

In our 24-hour society, the accurate tracking of time now impacts on every facet of our lives. Time is everywhere from our bedside tables to the computers we use for work and leisure. With clocks that only lose milliseconds over centuries, you would think that time is now well understood.

However, the study of time still holds a strong fascination with scientists and amateurs alike as they continue to search for an even more accurate way of tracking it. If you thought time was little more than a commodity that seems to slip through your fingers, read on to discover what time is, how we measure its passage, and how you can make sure you never miss a second.

Time is a genie that we have managed to successfully tame, but it can still amaze and astonish us. Scientists and amateurs alike continue to investigate time and attempt to develop even more accurate ways of measuring it to give the electronic systems we rely on more versatility and much more reliability.

There is no single global timepiece that all other clocks are set by. It's a myth that the atomic clock at Greenwich is used to set all other clocks. The reality is that around 40 labs across the world maintain 260 atomic clocks that all help to accurately track the world's various time zones.

In the UK, the National Physical Laboratory is responsible for accurate timekeeping. Since 2007, the Anthorn Radio Station has broadcast the time from the NPL's atomic clock across the UK to radio controlled clocks and watches, and the speaking clock.

Understanding time

The accurate tracking of time is an essential component of our lives. Without an agreed global time scale, human society simply couldn't function. You have to go back to 1884 at the Royal Observatory Greenwich astronomy to find the first recorded instance of international time.

The International Meridian Conference held that year decided that Greenwich would be used as the Prime Meridian that all other time zones would extrapolate from. Today, Greenwich Mean Time should be properly known as Universal Time 1 (UT1) and is a measurement of mean solar time.

Fast forward to 1958 where International Atomic Time (TAI) from the French Temps Atomique International was established. This tracks the time on the Earth's geoid, or its rotating surface, and forms the foundation of today's international time scale system known as Coordinate Universal Time (UTC) from the French Temps Universel Coordonne. We all use the UTC without knowing it, as it's the format (hours, minutes and seconds) your PC uses when it displays the time.

Gathering the time measurements from atomic clocks strategically placed around the world generates UTC. These measurements are used to produce the accurate measurement of the second as defined by the International System of Units (SI). The idea is to produce a time that is as close as possible to the definition of a second.

Just how long is a second?

This is currently based on the frequency of transmission between energy levels in a caesium atom. To be exact, the formal definition is: 'The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom', with the additional qualification in 1997 that, 'This definition refers to a caesium atom at rest at a temperature of absolute zero.'

Just to confuse things further, the time as tracked by TAI, UTC and UT1 don't match precisely because the atomic measurements taken by TAI and UTC don't take the rotation of the Earth into consideration. As we are familiar with UTC time and UT1, these two standards are synchronised together by adding or subtracting a leap second.